In a typical wireless communication system, baseband voice or data signals are carried on a radio frequency (RF) carrier frequency. In order to transmit signals on a RF carrier frequency, a transmitter is provided with circuitry that places the baseband signal on the RF carrier signal. This process is commonly termed frequency up-conversion and the circuitry that performs the process of converting from an intermediate frequency (IF) to a radio frequency (RF) is commonly referred to as a frequency up-converter. Likewise, at the receiving end, the baseband signal is extracted from the incoming RF signal picked up by the receiving antenna. This process is commonly termed frequency down-conversion and the circuitry that performs this process is commonly referred to as a frequency down-converter.
Frequency up conversion in such systems is typically performed by a mixer which multiplies the baseband signal to be transmitted, or the received RF signal, with a local oscillator (LO) signal to produce a signal that has spectral energy distributed at sums and differences of the LO signal and the baseband signal or the RF signal. In an up-conversion mixer the desired output is the sum of the local oscillator frequency flo and the baseband signal frequency flq while in a down conversion mixer, the desired output is the difference between the local oscillator frequency flo and the incoming signal frequency frf 
In some mixing applications the local oscillator may be a non-sinusoidal wave such as a square wave containing spectral energy located at a fundamental frequency and at harmonic frequencies of the fundamental frequency. This leads to harmonic mixing in the mixing whereby the mixing of such a LO signal with the baseband signals generates signals with spectral energy at flq away from the harmonics of the LO signal as well as the desired signal.
Some wireless systems use zero intermediate frequency architecture, referred to herein as zero-IF architecture in which a signal is up-converted in mixing circuitry, in a single step, directly from quadrature related in-phase (I) and quadrature (Q) signals at baseband frequencies. In some devices Low-IF technology is used in which an RF signal is mixed up from a non-zero low or moderate intermediate frequency, typically a few megahertz
FIG. 4 is a block diagram of an exemplary zero-IF quadrature direct frequency up-converter of a transceiver. A quadrature baseband signal comprising two components BBI and BBQ is fed, from two IQ digital to analogue converters (DAC) (not shown) through first and second low pass filters 11, 12 in order to filter the DAC aliases. The outputs of the low pass filters 11, 12 are then fed via buffers 15 and 16 to first and second mixers 21, 22, respectively, where they are mixed with the two quadrature components LOI and LOQ of a quadrature local oscillator signal to up-convert the baseband signal. The buffers 15 and 16 each comprise an amplifier and have a low output impedance to provide current to a PPA when the mixers are switching. The Low pass filters 11, 12 have a high output impedance. The frequency of the local oscillator signal corresponds to the desired RF transmission frequency. The outputs of the two mixers 21, 22 are then fed to a pre-power amplifier (PPA) 31 to amplify the RF signal. The output of the amplifier 31 is fed to an inductor and an optional balun for differential to single ended conversion prior to being transmitted by an antenna. Graph (i) of FIG. 5 illustrates the tones generated at the input to the PPA 31. Tone 3flo-flq, is the product of the harmonic mixing in the mixer. Typically the radio to desired signal is 10 dBc. Graph (ii) of FIG. 5 illustrates how the 3rd order intermodulation 3f10−fIQ−2*(flo+flQ) creates parasitic tones flo−3*flq 
In order to improve the linearity of the transceiver, the 3rd order intermodulation product of the 3× LO frequency at the mixer output with the desired signal should be reduced. A number of attempts have been made to improve the linearity of the PPA, to decrease the level of 3×LO at the PPA input
In some approaches by playing with biasing, input level and transistors sizing, it is possible to improve the linearity of the PPA. However, for cellular applications, it can be very difficult to improve the linearity by 15 dB without having a strong impact on noise and power consumption.
Another proposed technique to improve linearity is by inserting intermediate filtering between the mixer and the PPA. This solution however has the drawback that there is a compromise between desired attenuation and 3×LO filtering. Moreover, this type of solution requires calibration during manufacture.
Instead of using a standard square wave at the LO side of the mixer, some approaches such as described in WO 2008/135954 employ specially generated LO waveforms. A composite signal with 3×LO and 5×LO harmonics cancellation is used in some approaches. An example of a composite signal is illustrated in FIG. 5 where a first LO signal LO1 and a second LO signal LO2 have equal and opposite 3rd and 5th order harmonics creating a composite signal LO1+LO2 which is free of 3rd and 5th harmonics. Some techniques apply this principle in current mode mixers. The main drawbacks of this procedure are that the noise/linearity compromise of such a current mode mixer requires a large biasing current, and the baseband current has to be switched off during some phases.